I have a spare 3 Pounds on the eShop, and there happens to be a sale on the NES Megaman games on the 3DS eShop.

Which one of them do you recommend me to get?

Get more monies, and buy all.

Mega Man 3, 2, 1, 5, 6, and 4 in that order.

Here's a video of mine playing Mega Man 3. https://www.youtube.com/watch?v=MJT94JO883E

Kinda tight on cash right now. I wasn't even planning on buying any Mega Man games, but I had some leftover change.

Mega Man 2 of course! The only Mega Man game which I wouldn't recommend is Mega Man 4! It's so damn annoying at times and the Robot Masters are pretty bleh!

So it's 3 then? :P

Three is my favorite of the original series yeah, but if by chance you can purchase Mega Man X on t he Wii U and  you can afford it I'd recommend that instead.

Already played all the SNES and beyond Mega Man games, I just never played the original NES ones.

Mega Man 4, favourite of the classic series for me, all the stages are fun (no crap like Quick Man lasers or the Doc Robot stages), the robot master weapons are fun to use and the music is great as always. My second choice would be Mega Man 2.

2 > 3 > 5 > 4 > 6 > 1

If you're frugal / not the biggest Megaman fan, just buy 2, 3 and 5.